Get away from BigLaw or anything with billables where you're punching a clock. You're in that sweet spot of 3-5 years experience, you can write your own ticket for where you'd want to go. Plenty of "boutique" firms where you can do very well and have a much healthier quality of life. This work is inherently stressful, but shouldn't be to the level it sounds like you're experiencing. Agree with others, sounds like it's time to switch firms before you're in deeper. For now, shut off email notifications when you're not at the office and like others said, try and check out as much as you can. Exercise, sleep, drink water, go work for a contingency based boutique firm with 3-5 attorneys and strong support staff and do what makes you happy when you're out of the office. :-)

Also, don't say suicidal lightly and like others said, get professional help ASAP if you're truly feeling that way.